Reactions of coordinated ligands: coupling of two diphosphenes (P:P) at a metal center to give a coordinated phosphometallocyclopentane. Synthesis and x-ray structure of cyclic (tert-BuPP-tert-Bu)Ni(tert-BuP-tert-BuP-tert-BuP-tert-BuP): a square-planar nickel(II) geometry imparted by sterically demanding ligands